1. Field of the Invention
The present invention relates in general to digital radio communication and, in particular, to a method and apparatus for synchronizing discrete frames of data transmitted as a continuous stream of digital data in a receiver.
2. Background Art
The transmission of digital voice data over a radio frequency communication link is very often implemented by transferring the stream of digital data as discrete packets or frames (as opposed to a continuous homogeneous stream) of information from a transmitter to a receiver. Proper recovery of this digital information depends in part upon the receiver being in sync with the stream of incoming digital data packets. In general, apparatuses for synchronizing frames within a stream of digital data have been known in the art. Most of these prior art apparatuses provide for synchronization occurring at a particular repeating interval during the reception of an incoming digital data stream.
In some digital communication systems, multiple types (configurations) of packets each having different characteristics, such as length and data type, are employed. In such a system, one packet type may configured for voice, another for command, and yet another to acquire and maintain synchronization between the transmitter and receiver. A system incorporating multiple data packet types may often experience control problems which typically increases system complexity and contributes to other undesirable design and operating characteristics. Cordless telephones designed to the CT-2 specification are representative of this type of system.
In devices following the CT-2 specification, after initiation of the radio link, the ratio of transmitted audio data to command data is quite large resulting in limited ability to control and monitor the radio link during an on-going conversation. The CT-2 specification introduces further complexity by not utilizing a synchronization field during this on-going conversation, instead establishing synchronization only during the initiation of the radio link Accordingly, it is an object of the present invention to provide a single data frame type wherein each frame contains a synchronization field, an audio field and a command data field in every frame.
Some prior art digital communication systems do already use packets having segregated fields for voice, command and error correction data in each frame. This approach creates an inherent delay in audio data transmission and processing caused by the arrangement of the frames. First, in many such prior art systems the segregation of the various types of data (i.e. voice, command, sync) requires extensive buffering to bundle the packet for transmission and, later, unbundle the overall frame. Second, the addition of data extraneous to the actual voice data causes audio delay. Delay is undesirable in a digital audio path because it decreases the Mean Opinion Score (a commonly used assessment of the quality of the communication link as it relates to speech) for the overall transmission system.
In some of these systems a synchronization marker or field is included in the stream or as part of a packet at regular intervals. Further adding to the delay, this sync field is located within the incoming stream of data by the synchronizing apparatus in the receiver to achieve sync. The design of this synchronization field requires balancing of various opposing factors. The loner the synchronization field the more likely "location" of that field has truly located the sync word. However, the longer the synchronization field, the greater the delay introduced into the audio channel. Conversely, the shorter the sync field the less likely "location" of that field is truly the sync word rather than "look-alike" data, yet the shorter the introduced audio delay.
Accordingly, it is a further object to include a specifically formatted synchronization field in each data frame to simplify synchronization of the packets. However, since introduction of a synchronization word adds delay, it is an associated object of the present invention to provide a hybrid synchronization scheme that minimizes delay in the audio path.
This and other objects of the present invention will become obvious to those of ordinary skill in the art based upon the present specification, drawings and claims.